Magnetic bearing for suspending a rotating shaft using high Tc superconducting material

ABSTRACT

A magnetic bearing magnetically suspends a rotating shaft within a stator. The magnetic bearing has a first bearing element rigidly linked with the shaft which is enclosed by a second bearing element pertaining to the stator, thereby defining a bearing clearance. A system of magnets of the first bearing element with permanent magnet elements and a cooled superconducting system of the second bearing element with high T c  superconducting material are commonly enclosed by at least one insulating compartment. An additional compartment which is separate from the insulating compartment encloses the bearing clearance and partial compartments that radially extend on lateral sides of the superconducting system and of the system of magnets up to the shaft and are sealed from the shaft.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/DE0/03173 filed on 20, Aug. 2001 and GermanApplication No. 100 42 962.9 filed on 31, Aug. 2000.

BACKGROUND OF THE INVENTION

The invention relates to a magnetic bearing where a shaft which canrotate is borne magnetically within a stator. The magnetic bearing isintended to have the following features:

a first bearing part is rigidly connected to the shaft and is surroundedby a second bearing part, which is associated with the stator, forming abearing gap between these bearing parts,

the first bearing part contains a magnet arrangement with permanentlymagnetic elements,

the second bearing contains a superconducting arrangement with ahigh-T_(c) superconductor material, with magnetic bearing forces beingproduced between the superconducting arrangement and the permanentlymagnetic elements of the magnet arrangement, and

a cooling apparatus is provided for cooling the superconductor materialof the superconducting arrangement to an operating temperature below thecritical temperature of the superconductor material.

A magnetic bearing such as this is disclosed in DE 44 36 831 C2.

Magnetic bearings allow moving parts to be provided with bearings whichmake no contact and are therefore free of wear. They require nolubricants and can be constructed to have low friction. In this case, abody which can rotate (rotating body) can be hermetically sealed, thatis to say in a vacuum-tight manner, from the outer area surrounding it.

Known magnetic bearings use magnetic forces between stationaryelectromagnets on a stator and ferromagnetic elements, which are on arotor body and rotate with it. With this type of bearing, the magneticforces are always attractive. In consequence, it is in principleimpossible to achieve a bearing which is inherently stable in all threespatial directions (see “Eamshaw's Theorem” in “Trans. Cambridge Phil.Soc.”, Vol. 7, 1842, pages 97 to 120). Magnetic bearings such as thesetherefore require active bearing control, which uses position sensorsand a control loop to control the currents and the supporting magnetsand to counteract any discrepancies of the rotor body from a nominalposition. The control process, which needs to have a plurality ofchannels for this purpose, requires complex power electronics. Inaddition, a mechanical emergency bearing must be provided as aprecaution against sudden failure of the control loop. Correspondingmagnetic bearings are used, for example, in turbo-molecular pumps,ultra-centrifuges, high-speed spindles for machine tools, and X-raytubes with rotating anodes; use for motors, generators, turbines andcompressors is likewise known.

In principle, superconductors allow a new type of magnetic bearing: oneof the bearing parts is in this case formed with permanently magneticelements which induce shielding currents in the event of a positionchange, as a consequence of field changes in the superconductor materialof a further, second bearing part which surrounds the first bearing partwith a gap. The forces which result from this may be repulsive orattractive, but are directed such that they counteract the deflectionfrom a nominal position. In contrast to known magnetic bearings, it ispossible in this case to achieve an inherently stable bearing (see, forexample, “Appl. Phys. Lett.”, Vol. 53, No. 16, 1988, pages 1554 to1556). In contrast to known magnetic bearings, there is no need here forany complex control system that is susceptible to defects; however, acooling apparatus must be provided in order to cool the superconductormaterial to an operating temperature below the critical temperature ofthe superconductor material.

Appropriate superconducting bearing parts for magnetic bearings such asthese may be one of the first fields of use for the metal-oxidehigh-T_(c) superconductor materials which have been known since 1987,such as those based on the Y-Ba-Cu-O material system, which can becooled to an operating temperature of about 77 K using liquid nitrogen.

Use of appropriate high-T_(c) superconductor material is envisaged forthe magnetic bearing which is disclosed in the DE-C2 document citedinitially. The magnetic bearing contains a large number of permanentlymagnetic elements which are in the form of annular discs and are locatedone behind the other in the axial direction on a rotor shaft. Theseelements are polarized such that the polarization alternates when seenin the axial direction of the shaft. Comparatively thin ferromagneticintermediate elements are arranged in each case between adjacentelements. These intermediate elements primarily have the task ofmagnetically concentrating the magnetic lines of force of adjacentpermanently magnetic elements, so that a particularly high magneticfield strength is produced on the side of each intermediate elementwhich faces the bearing gap. This bearing part of the rotor body,together with its magnet arrangement composed of permanently magneticelements, is surrounded by a fixed-position bearing part of a stator.This bearing part contains a superconducting arrangement with ahigh-T_(c) superconductor material such as Yba₂Cu₃O_(x), with theabovementioned magnetic bearing forces being produced between thesuperconducting arrangement and the permanently magnetic elements of themagnetic arrangement. The superconductor material of the conductorarrangement is kept at about 77 K by liquid nitrogen (LN₂). For thispurpose, cooling channels through which this coolant is passed areprovided on the outside of the superconducting arrangement.

In the case of magnetic bearings in which parts which need to becryogenically cooled are adjacent to the bearing gap, one problem thatcan occur is that environmental air can reach the cold componentsthrough the bearing gap, with the moisture in the air freezing there.Corresponding icing can lead to functional restrictions or damage to thebearing. In the case of the magnetic bearing which is disclosed in theabovementioned DE-C2 document, such icing of the bearing gap can beavoided by emitting vaporizing nitrogen. The necessary cooling power forthe bearing is in this case from a few watts up to the order ofmagnitude of 10 W at 50 to 80 K. However, if other cooling techniquesthan those used for the known magnetic bearings are envisaged,especially using so-called cryogenic coolers with only indirect cooling,there is no corresponding capability to avoid the risk of icing in thebearing gap, since no vaporizing coolant gas is then available.

SUMMARY OF THE INVENTION

One possible object of the present invention is therefore to refine themagnetic bearing having the features mentioned initially, such that suchrisk of bearing icing is minimized irrespective of the chosen coolingtechnique, and such that the sealing complexity can be kept low.

This object may be achieved in that, in the case of the magnetic bearinghaving the features mentioned initially, the superconducting arrangementand the magnetic arrangement are also jointly surrounded by at least oneisolation area, and in that an additional area is provided, which isseparated from the at least one isolation area and comprises the bearinggap and subareas which extend on side end faces of the superconductingarrangement and of the magnet arrangement radially as far as the shaftand are sealed there with respect to the shaft.

The advantages which are associated with this embodiment of the magneticbearing are, in particular, that the complexity for sealing theadditional area from the parts which can rotate can be kept low. This isbecause the seal uses the smallest possible diameter, so that thecircumferential speed of the parts of the seal which also rotate isminimized. This makes it simpler for the seal to operate, andcorrespondingly lengthens its life. The simplified sealing, which maythus also be designed to be effective, of the additional area alsoresults in the risk of ingress of gases which can freeze at leastlargely being avoided.

The additional area of the magnetic bearing can thus be evacuated in asimple manner. This advantageously allows friction losses to be reduced.In the event of any leakage of the sealant on the shaft, a small amountof air could admittedly in theory enter, however, severe icing iscounteracted by the fact that, in this case, even a defective seal stillprovides a major impediment to the exchange of air, especially becausethe corresponding flow cross sections of the side subareas can be keptsmall. The magnetic bearing therefore has good emergency runningcharacteristics.

Instead of this, it is particularly advantageous to fill the additionalarea with a dry barrier gas. Any gas or gas mixture which has nocomponents that freeze at the operating temperature in the area of thebearing gap is suitable for use as the dry barrier gas. Appropriatebarrier gases can be chosen from the group of helium, neon, argon ornitrogen, with a gas mixture having at least one of these gases alsobeing suitable. If the additional area is filled with a gas, then thetemperature decreases from the hot shaft toward the cold bearing gap inthe side subareas, which can advantageously be formed with a small crosssection, without any thermal losses occurring due to convection. This isbecause convection is avoided by the hot end of the side subareas filledwith gas whose density is less being located closer to the shaft. Thecentrifugal force then results in stable layering during rotation.Furthermore, it should be regarded as being particularly advantageousthat operation is also possible with slightly fluctuating pressures inthe gas area, so that gas losses due to leakages, for example in thesealing area, are tolerable within wide limits. Depending on therequirement for the applications, the gas pressure may be below 1 bar,around 1 bar or more than this, so that especially in the latter caseentry of moist air with ice formation in the cold area is reliablyprevented.

In order to provide effective thermal isolation for those parts of themagnetic bearing which need to be cooled, it is possible, in particular,to evacuate the at least one isolation area. Instead, or preferably inaddition to this, this area can also at least partially be filled withat least one isolation means, which is known per se.

It should also be regarded as particularly advantageous that the coolingapparatus for the magnetic bearing has at least one cryogenic coolerwith at least one cold head. This cold head is then thermally coupled tothe superconducting arrangement, preferably via at least one thermallyconductive body, in order to provide indirect cooling for thesuperconducting arrangement. The use of such a cryogenic cooler has theadvantage that the cooling power is available virtually at the push of abutton, and there is no need to handle cryogenic liquids. In this case,indirect cooling by thermal conduction to the cold head is sufficientfor effective cooling of the known high-T_(c) superconductor materials.If a cryogenic cooler is used, it is admittedly not possible to preventicing of the bearing gap by the emergence of vaporizing cooling gas suchas nitrogen. Furthermore, thermal isolation of the superconductormaterial in the bearing gap would result in considerably enlargement ofthe bearing gap and would thus also result in a corresponding drasticreduction in the supporting force and stiffness of the bearing. For goodoperation, the bearing gap should, however, be as small as possible and,for example, should be in the order of magnitude of 1 mm. If the bearingwere to be placed completely in an isolating vacuum vessel for thispurpose, then this in principle would have to be sealed from therotating shaft by two hermetic seals. This would in fact have thedisadvantage that the vacuum would break down in the event of leakage,correspondingly interfering with the operation of the bearing and of themachine parts being borne.

In this case, the permanently magnetic elements would then be cooleddown slowly essentially by heat radiation to an intermediate temperaturebetween the operating temperature of the superconductor material and theoutside temperature. The configuration of the special additional area,which is separated from the isolation area which jointly surrounds thesuperconducting arrangement and the magnetic arrangement,advantageously, however, allows the corresponding sealing problems to beovercome since—as already stated—the sealing of the additional area issubject to considerably less stringent requirements. Even indirectcooling can therefore also be used without any problems.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 shows a longitudinal section through the magnetic bearing, and

FIG. 2 shows a detailed view of a sealing apparatus for the magneticbearing as shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

Corresponding parts are provided with the same reference symbols in thefigures.

The magnetic bearing, which is annotated in general by 2 in FIG. 1, isbased on an embodiment as is disclosed in the cited DE 44 36 831 C2. Thebearing is intended to be used as a magnetic bearing for a rotor shaft 3(which can rotate), which may be composed of a nonmagnetic material suchas an appropriate steel. For example, the shaft 3 is part of anelectrical machine such as a generator, which is not shown in thefigure. It has an associated first bearing part 5, which rotates with itand surrounds it concentrically in the bearing area. This bearing partis mounted rigidly on the shaft 3 via mounting elements 6 a and 6 b,which are in the form of discs and may advantageously be composed ofpoorly thermally conductive material such as GFC, for minimal heatintroduction. According to the cited DE-C2 document, the first bearingpart 5 contains a magnet arrangement 7 with elements 8 i which are inthe form of annular discs and are composed of permanently magneticmaterial. These elements are magnetically polarized alternately whenseen in the axial direction and are separated from one another byintermediate elements 9 i which are in the form of annular discs and arecomposed of ferromagnetic material such as iron. The ferromagneticmaterial of these intermediate elements is used to concentrate themagnetic flux on the cylindrical outer surface of the first bearing part5, thus increasing the supporting force of the magnetic bearing. All theelements 8 i and 9 i are arranged in the form of a stack one behind theother in a supporting body 10, which ensures the drive connection to theshaft 3 via the mounting elements 6 a and 6 b which are in the form ofdiscs.

In the first bearing part 5, which rotates with the shaft and has thepermanently magnetic elements 8 i, is separated by an air gap 12,surrounded by a second hollow-cylindrical, fixed-position bearing part13, in which case the gap width w may be in the order of magnitude of afew millimeters. The fixed-position bearing part 13 which forms a statorhas a hollow-cylindrical superconducting arrangement with one of theknown high-T_(c) superconductor materials on its inside facing the firstbearing part 5, which can be kept at an operating temperature below itscritical temperature during operation. When the position of thepermanently magnetic elements 8 i changes, this results in field changeswhich induce shielding currents in this superconductor material, whichlead to the desired magnetic bearing forces between the bearing parts 5and 13.

The hollow-cylindrical superconducting arrangement 14 of thefixed-position second bearing part 13 is mounted within a supportingbody 16 on its side facing away from the bearing gap 12 via anintermediate cylinder 15 composed of a thermally highly conductivematerial such as copper. In order to cool it, the superconductingarrangement is thermally coupled via a thermally conductive body 18 to acold head 20 of a cryogenic cooler, which is not shown in any moredetail. Appropriate cryogenic coolers have a closed compressed gascircuit in particular for helium gas and are, for example, of theGifford-McMahon or Stirling type, or are in the form of so-calledpulsed-tube coolers. Appropriate cryogenic coolers are generally known.They may in particular be in the form of so-called regenerative coolers(based on the normal classification of cryogenic coolers) with aregenerator or a regenerative operating cycle (see the “Proceedings16^(th) Int. Cryog. Engng. Conf. (ICEC 16)”, Kitakyushu, JP,20.-24.05.1996, Verlag Elsevier Science, 1997, pages 33 to 44; “Adv.Cryog. Engng.”, Vol. 35, 1990, pages 1191 to 1205 or U.S. Pat. No.5,335,505).

The cold head 20 is located on the outside of a bearing housing 21,which completely surrounds the bearing 2 and is at room temperature. Thefixed-position bearing part 13 is mounted on the inside of this bearinghousing via mounting elements 22 a and 22 b which are in the form ofannular discs. In order to ensure that a minimum amount of heat isintroduced, these mounting elements are preferably composed of a poorlythermally conductive material such as GFC. For thermal insulationreasons, at least one part of the interior of the housing 21 is in theform of at least one thermal isolation area, at least the majority ofwhich surrounds the unit comprising the superconducting arrangement 14and the magnet arrangement 7. In particular, as assumed in the followingtext, this area may be evacuated. Instead of this, or advantageously inaddition to it, this area may be at least partially filled with at leastone thermal isolation substances. Suitable isolation substances are, forexample, insulating foam, super insulation, insulating flakes or glassfibers. According to the illustrated embodiment, two vacuum areas V1 andV2, which are separated from one another, should be provided for thermalisolation. In this case, the vacuum area V1 comprises two side areas V1a and V1 b and a radially externally located area V1 c, which is locatedbetween the fixed-posibon second bearing part 13 and the radiallyexternal part of the vacuum housing 21. The vacuum area V2 is locatedbetween the shaft 3 and the first bearing part 5 which rotates with it,and is bounded at the side by the mounting elements 6 a and 6 b. It thusrotates with these parts. In contrast to the chosen illustration, thevacuum areas V1 and V2 may also be at least partially filled with one ofthe thermal isolation substance which are known per se, such as superinsulation or insulation foam.

There is advantageously no need to seal the vacuum area V1 from therotating shaft 3 in one refinement of the magnetic bearing. This isbecause, the bearing gap 12 which is formed between the fixed-positionsecond bearing part 13 and the first bearing part 5 which rotates withthe shaft and has a narrow gap width w should not be connected to thevacuum area V1. Rather, the intermediate area of the bearing gap shouldbe part of a (further) additional area 25, which is sealed on the shaft.For this purpose, the area of the bearing gap 12 at its axial side endsin each case opens into subareas 25 a and 25 b which extend radially asfar as the shaft 3. These subareas, which each contain a space in theform of an annular disc, are advantageously kept very narrow in theaxial direction. On the shaft, they merge into sealing gaps 26 a and 26b which lead axially to the exterior and are parts of sealing apparatus28 a and 28 b, respectively. The additional area 25 may be evacuated ormay be filled with a dry barrier gas, such as nitrogen. The gas pressureis generally between 0.1 and 10 bar, with a pressure which is slightlygreater than the normal pressure being particularly advantageous. Thesealing apparatuses 28 a and 28 b are not shown in any more detail inFIG. 1. Details of them can be seen in FIG. 2.

As can be seen from the detailed illustration in FIG. 2, with regard toone of the sealing apparatuses, for example with regard to the apparatus28 a, the associated sealing gap 26 a opens into the outer area R whichsurrounds the bearing housing 21 with the magnetic bearing 2, and isgenerally at room temperature and normal pressure. In order to provide aseal from this outer area, at least one sealing element 30 of thesealing apparatus 28 a is located in the sealing gap 26 a and is in theform, for example, of a ferrofluid sealing element (see DE 20 34 213 A).The sealing apparatus 28 a therefore comprises a permanent magnet 33with magnet poles N and S as well as two yoke limbs 34 a and 34 b whichare fitted to them at the sides and which carry the magnetic flux. Theyoke limbs are provided with points in the area of the sealing gap 26 aon their side facing the shaft, on each of which a ferrofluid ring 35 iis held magnetically. The shaft 3 must be ferromagnetic for thispurpose, at least in the area of the respective sealing element 30. Forexample, for this reason, a hollow tube which is not shown in the figurethat is composed of ferromagnetic material is pushed over the shaft,which is manufactured from nonmagnetic material.

Instead of this indicated type of sealing apparatus, other knownembodiments such as labyrinth seals or gap seals may also be used. Anappropriate seal is provided for sealing apparatus 28 b with its sealinggap 26 b.

As can also be seen from the detailed illustration in FIG. 2, the vacuumarea V1 or its subarea V1 a is terminated in the area of the shaft 3 bya fixed-position wall 31 of the sealing gap 26 a and by the yoke limb 34a. The yoke limb in this case represents a lengthened part of the sidewall 21 a of the bearing housing 21. However, instead of this, it isalso advantageously possible to allow the side wall 21 a to run as faras the wall 31 and to fit the sealing apparatus 28 a to the side wall 21a at the side, for example by flange-connecting in a vacuum-tight mannerby an O-ring.

One advantageous feature is that none of these embodiments of themagnetic bearing result in any sealing problems between the isolatingvacuum area V1 or isolation area and rotating parts. All that istherefore required from the bearing is that the bearing gap 12 to theshaft 3 be sealed in a manner which is less problematic and is lesscomplex.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

What is claimed is:
 1. A magnetic bearing to magnetically support a shaft for rotation within a stator, comprising: a first bearing part rigidly connected to the shaft; a second bearing part associated with the stator, the second bearing part surrounding the first bearing part with a bearing gap between the first and second bearing parts; a magnet arrangement with permanently magnetic elements, provided in the first bearing part; a superconducting arrangement with a high-T_(c) superconductor material, provided in the second bearing part, the superconducting arrangement and the permanently magnetic elements of the magnet arrangement creating magnetic bearing forces therebetween; a cooling apparatus to cool the superconductor material of the superconducting arrangement to an operating temperature below a critical temperature of the superconductor material; subareas which extend on side end faces of the superconducting arrangement and of the magnet arrangement radially as far as the shaft and which are sealed there with respect to the shaft; and at least one isolation area jointly surrounding the superconducting arrangement and the magnet arrangement and separated from the bearing gap and the sub areas.
 2. The magnetic bearing as claimed in claim 1, wherein the bearing gap and the subareas are filled with a dry barrier gas.
 3. The magnetic bearing as claimed in claim 2, wherein the barrier gas is at least one gas selected from the group consisting of helium, neon, argon and nitrogen.
 4. The magnetic bearing as claimed in claim 2, wherein the barrier gas is at a pressure of between 0.1 and 10 bar.
 5. The magnetic bearing as claimed in claim 3, wherein the barrier gas is at a pressure of between 0.1 and 10 bar.
 6. The magnetic bearing as claimed in claim 5, wherein the bearing gap and the subareas are evacuated.
 7. A magnet device as claimed in claim 6, wherein the at least one isolation area is evacuated.
 8. The magnet device as claimed in claim 7, wherein the at least one isolation area is at least partially filled with at least one isolation substance.
 9. The magnetic bearing as claimed in claim 8, wherein the at least one isolation substance is selected from the group consisting of insulating foam, superinsulation, isolating flakes and glass fibers.
 10. The magnet device as claimed in claim 9, further comprising: a bearing housing at room temperature; and poorly thermally conductive mounting elements provided within the bearing housing to mount the superconducting arrangement.
 11. The magnetic bearing as claimed in claim 10, further comprising poorly thermally conductive mounting elements to mount the magnet arrangement on the shaft.
 12. The magnetic bearing as claimed in claim 11, wherein the at least one isolation area comprises a closed isolation area which bounds the first bearing part and the shaft and rotates.
 13. The magnetic bearing as claimed in claim 12, wherein the magnetic bearing further comprises a sealing apparatus arranged on each side of the shaft, and the subareas each open into an axial sealing gap which surrounds the shaft and is connected to a corresponding sealing apparatus.
 14. The magnetic bearing as claimed in claim 13, wherein each sealing apparatus has at least one seal selected from the group consisting of ferrofluid seals, labyrinth seals and gap seals.
 15. The magnetic bearing as claimed in claim 14, wherein the cooling apparatus has at least one cryogenic cooler with at least one cold head.
 16. The magnetic bearing as claimed in claim 15, wherein the cryogenic cooler is selected from the group consisting of a Gifford-McMahon cooler, a Stirling cooler, and a pulsed-tube cooler.
 17. The magnetic bearing as claimed in claim 16, wherein the cold head is thermally coupled to the superconducting arrangement via at least one thermally conductive body.
 18. The magnetic bearing as claimed in claim 1, wherein the bearing gap and the subareas are evacuated.
 19. A magnet device as claimed in claim 1, wherein the at least one isolation area is evacuated.
 20. The magnet device as claimed in claim 1, wherein the at least one isolation area is at least partially filled with at least one isolation substance.
 21. The magnetic bearing as claimed in claim 20, wherein the at least one isolation substance is selected from the group consisting of insulating foam, superinsulation, isolating flakes and glass fibers.
 22. The magnet device as claimed in claim 1, further comprising: a bearing housing at room temperature; and poorly thermally conductive mounting elements provided within the bearing housing to mount the superconducting arrangement.
 23. The magnetic bearing as claimed in claim 1, further comprising poorly thermally conductive mounting elements to mount the magnet arrangement on the shaft.
 24. The magnetic bearing as claimed in claim 1, wherein the at least one isolation area comprises a closed isolation area which bounds the first bearing part and the shaft and rotates.
 25. The magnetic bearing as claimed in claim 1, wherein the magnetic bearing further comprises a sealing apparatus arranged on each side of the shaft, and the subareas each open into an axial sealing gap which surrounds the shaft and is connected to a corresponding sealing apparatus.
 26. The magnetic bearing as claimed in claim 25, wherein each sealing apparatus has at least one seal selected from the group consisting of ferrofluid seals, labyrinth seals and gap seals.
 27. The magnetic bearing as claimed in claim 1, wherein the cooling apparatus has at least one cryogenic cooler with at least one cold head.
 28. The magnetic bearing as claimed in claim 27, wherein the cryogenic cooler is selected from the group consisting of a Gifford-McMahon cooler, a Stirling cooler, and a pulsed-tube cooler.
 29. The magnetic bearing as claimed in claim 27, wherein the cold head is thermally coupled to the superconducting arrangement via at least one thermally conductive body. 